Widely wavelength tunable integrated semiconductor device and method for widely wavelength tuning semiconductor devices

ABSTRACT

Alternative laser structures, which have potentially the same tuning performance as (S)SG-DBR and GCSR lasers, and a fabrication process which is similar to that of the (S)SG-DBR laser, are presented. The advantage of these structures is that the output power does not pass through a long passive region.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 09/573,794,filed on May 16, 2000, now U.S. Pat. No 6,728,279. U.S. patentapplication Ser. No. 09/573,794 claims priority benefits to EuropeanPatent Application No. 99870105.6, filed on May 17, 1999. U.S. patentapplication Ser. No. 09/573,794 also claims priority benefits to U.S.Provisional Patent Application No. 60/155,386, filed on Sep. 22, 1999.This application incorporates by reference U.S. patent application Ser.No. 09/573,794, filed on May 16, 2000 and U.S. Provisional PatentApplication No. 60/155,386, filed on Sep. 22, 1999 in their entirety.

FIELD OF THE INVENTION

The invention relates to multi-section integrated semiconducting devicesor lasers, comprising resonator sections, being either distributedreflection or transmission sections. The invention also relates tomethods for widely wavelength tuning semiconductor devices or lasers.

BACKGROUND OF THE INVENTION

Tuning of a conventional Distributed Bragg Reflector (DBR) semiconductorlaser is limited by the fact that the relative tuning range isrestricted to the relative change in the refractive index of the tuningregion. This means that the tuning range, under normal operatingconditions, cannot exceed 10 nm. This is substantially less than thepotential bandwidth, restricted by the width of the gain curve, which isabout 100 nm. Such conventional DBR lasers can functionally becharacterized as comprising a first part being a two-sided activesection, for creating radiation, for instance a light beam, byspontaneous emission over a bandwidth around one center frequency. Saidfirst part also guides said radiation or light beam. Such conventionalDBR lasers further have two reflectors. Said reflectors are boundingsaid two-sided active section, thus one at each side.

The limited selectivity problem has been recognized by Wolf, et al(European Transactions on Telecommunications and Related Technologies, 4(1993), No. 6) showing in FIG. 10 a laser structure with two parallelwaveguides but without gratings. These two parallel waveguides cannot beconsidered as resonators, indeed the spectra (shown in FIG. 10 b and c)show the comb mode spectra corresponding to arms B and A, but they areeither the comb mode spectra of arm B, the gain section and thereflectors R or the comb mode spectra of arm A, the gain section and thereflectors R. As the spacing between the spectral lines are determinedby the length of the structures, it appears that said spacing is stillvery small, resulting in still a low selectivity and a low tuneability.

Over the past years several advanced laser structures have been proposedwith an extended tuning range. Examples are the Y-laser [M. Kuznetsov,P. Verlangieri, A. G. Dentai, C. H. Joyner, and C. A. Burrus, “Design ofwidely tunable semiconductor three-branch lasers,” J. LightwaveTechnol., vol. 12, no. 12, pp. 2100–2106, 1994], the co-directionallycoupled twin-guide laser [M.-C. Amann, and S. Illek, “Tunable laserdiodes utilizing transverse tuning scheme,” J. Lightwave Technol., vol.11, no. 7, pp. 1168–1182, 1993], the Sampled Grating (SG) DBR laser [V.Jayaraman, Z. M. Chuang, and L. A. Coldren, “Theory, design andperformance of extended tuning range semiconductor lasers with sampledgratings,” IEEE J. Quantum Electron., vol. 29, no. 6, pp. 1824–1834,1993], the Super Structure Grating (SSG) DBR laser [H. Ishii, H. Tanobe,F. Kano, Y. Tohmori, Y. Kondo, and Y. Yoshikuni, “Quasicontinuouswavelength tuning in super-structure-grating (SSG) DBR lasers,” IEEE J.Quantum Electron., vol. 32, no. 3, pp. 433–440, 1996] and the Gratingassisted Coupler with rear Sampled Reflector (GCSR) laser [M. Öberg, S.Nilsson, K. Streubel, L. Bäckbom, and T. Klinga, “74 nm wavelengthtuning range of an InGaAsP/InP vertical grating assisted codirectionalcoupler laser with rear sampled grating reflector,” IEEE Photon.Technol. Lett., vol. 5, no. 7, pp. 735–738, 1993]. In the first twotypes of devices, a trade-off had to be made between the tuning rangeand the spectral purity (broad tuning range vs. high Side ModeSuppression Ratio (SMSR)). Therefore recently most research attentionhas gone to the (S)SG-DBR and GCSR lasers.

A sampled grating DBR laser, comprises of two sampled gratingsexhibiting a comb-shaped reflectance spectrum, with slightly differentpeak spacing due to the different sampling periods. As an alternative,other grating shapes can be used: these are normally referred to as“super structure gratings” (SSG). Lasers of this type have beenfabricated with tuning ranges up to about 100 nm. The operation of thedevice is such that through current injection in the two DBR sections, apeak of the front and rear reflectance comb are aligned at the desiredwavelength. The phase section is used to align a longitudinal cavitymode with the peaks of the two reflectors. The disadvantage of the(S)SG-DBR approach is that light coupled out of the laser has to pass along passive or inactive section, leading to loss. Also, the losses inthe two reflector sections increase with the amount of current injectedinto those sections, leading to a tuning current dependent output power.

The SG-DBR laser and the SSG-DBR laser are functionally characterized ascomprising a two-sided active region for light creation and tworeflectors one at each side of the active region, said reflectors havinga reflection characteristic with a plurality of reflection peaks. Saidcharacteristic has spaced reflection maxima points providing a maximumreflection of an associated wavelength. Such a characteristic can beobtained via sampled gratings, which exhibit a comb-shaped reflectionspectrum or via the so-called supergratings. Said gratings orsupergratings can also be characterized as distributed reflectors.

Sampled gratings can be described as structures in a waveguide system,having a periodically broken short-period structure including shortperiod stripped regions alternating with non-stripped regions. Thesupergratings can be described as structures in a waveguide systemhaving a diffractive grating having a plurality of repeating unitregions, each having a constant length, thus forming a modulationperiod, and at least one parameter that determines the opticalreflectivity of said diffractive grating varying depending on itsposition in each of said repeating unit regions along a direction ofoptical transmission in said laser, said diffractive grating extendingby at least two modulation periods. Reference is made to U.S. Pat. No.5,325,392 related to distributed reflector and wavelength tunablesemiconductor lasers, which is hereby incorporated by reference in itsentirety.

The SG-DBR laser and the SSG-DBR laser exploit constructive interferenceof the periodic characteristics of reflectors, located at differentsides of the active section, with different periodicity, to obtain awide tunability. The alignment of the reflector peaks can be describedby stating that the spacing of said reflective maxima points of thereflectors are different or are essentially not equal and only one saidreflective maxima of each of said reflectors is in correspondence with awavelength of said created lightbeam. Reference is made to patent U.S.Pat. No. 4,896,325, related to multi-section tunable lasers withdiffering multi-element mirrors, which is hereby incorporated byreference in its entirety.

As the construction of said reflectors leads to long inactive sections,this results in lasing output power losses.

Other lasers, which use a co-directional coupler, readily have a verywide tuning range, but there is insufficient suppression of neighbouringlongitudinal modes. The combination of a widely tuneable but poorlyselective co-directional coupler with a single (S)SG reflector will giveboth wide tuning and a good side mode suppression. Furthermore, theoptical output signal does not pass through a passive region. Againtuning of 100 nm has been achieved. Unfortunately, such a structure israther complicated to manufacture, requiring at least 5 growth steps.Reference is made to patent U.S. Pat. No. 5,621,828 related tointegrated tunable filters, which is hereby incorporated by reference inits entirety.

EP-A-0926787 describes a series of strongly complex coupled DFB lasers.In the disclosed structure, gratings are made within the activesections. Said gratings are selected such that no substantialinteraction between the lasers, defined by a grated active section, inseries is obtained. The disclosed structure enables generation ofmultiple wavelengths, even sumultaneously, but does not address theissue of selectivity and tuneability.

A parallel structure with a plurality of waveguides is disclosed in thePATENT ABSTRACT OF JAPAN, vol. 013, no. 026, 20 Jan. 1989, JP 63 229796(Fujitsu Ltd. The disclosed structure again enables radiation of aplurality of wavelengths but does not address the issue of tuneability.The optical switch is operated for selecting a waveguide, thus nosimultaneously optical connection between said waveguide is obtained.

AIM OF THE INVENTION

The aim of the present invention is to disclose laser structures whichare easy to manufacture and which are widely tuneable and have lowlasing output power losses.

SUMMARY OF THE INVENTION

In the present invention, alternative laser structures, apparatus ordevices are presented, which have potentially the same tuningperformance as (S)SG-DBR and GCSR lasers and which output power, and notpass through a long passive region.

An integrated/semiconductor tunable laser comprising a substrate made ofa semiconducting material, a two-sided active section on said substrate,and a plurality of sections on said substrate, is disclosed. Said lasercan be denoted as a multi-section integrated semiconductor laser. Saidactive section is radiation generating, for instance, but not limited tothe range of optical radiation. All said sections are connected to oneside of said active section. Note that this does not mean that they aredirectly coupled to said active section. In case of optical radiation,said connection can be denoted as an optical connection. At least two ofsaid sections include a waveguide system. Each of said sections definesa resonator.

These resonator sections have a spectra with spaced maxima resonantpoints themselves. They are themselves either a filter or a reflectorwith a comb mode spectra.

The resonators used in the present invention have resonantcharacteristics with a plurality of resonant peaks. Alternatively it canbe said that said resonators have spaced resonant maxima pointsproviding a maximum resonance of an associated wavelength. Thetransmission filters used in the present invention have a transmissioncharacteristic with a plurality of transmission peaks. Alternatively itcan be said that said transmission filters have spaced transmissionmaxima points providing a maximum transmission of an associatedwavelength. The reflectors used in the present invention have areflection characteristic with a plurality of reflection peaks.Alternatively it can be said that said reflectors have spaced reflectivemaxima points providing a maximum reflection of an associatedwavelength.

The spacing of said resonator maxima points corresponding to thetransmission or reflective maxima points of at least two of saidsections are selected to be essentially not equal or different. Saidlaser is therefore denoted as an integrated semiconductor laser withdifferent reflection or transmission sections. The transmission andreflection characteristic of said transmission filters and reflectorsare positioned relative to each other such that at least one of saidtransmission or reflective maxima of each of said two sections overlapeach other. This means that said sections have at least one transmissionor reflective maxima for a same first frequency. Due to the differentspacings of said transmission or reflective maxima points a small shiftof one of said transmission or reflective characteristics can result inoverlapping of at least one other transmission or reflective maxima ofeach of said two sections. Said sections have then at least one secondfrequency in common, which can be largely different from said firstfrequency. Said shift can be due to current injections in saidtransmission or reflective sections. It can be said that said lasercomprises means for injecting current into some of said plurality ofsections, resulting in said transmission or reflection characteristicbeing shifted in wavelength. Said overlapping maxima points define aplurality of lasing wavelengths. Due to the small shifting of at leastone resonator characteristic, said device jumps from a first set oflasing wavelengths to another set of lasing wavelengths. The spacings ofthe maxima resonant points are accordingly essentially determined by thegrating instead of the length of the sections.

It can be said that said active section creates a radiation or alightbeam by emission and that the device emits an emitted laser beamwith the wavelength of said emitted lightbeam being in correspondencewith said overlapping maxima of said transmission filters or reflectors.Said active section is thus creating radiation or a light beam byspontaneous emission over a bandwidth around a center frequency andguides said radiation or light beam and has (optical) amplificationactions. Said emitted radiation or lightbeam does not pass through saidplurality of sections. The combination of said plurality of sections,having a combinated reflection action, and said (optical) amplificationaction of said active section causes lasing at said set of lasingwavelengths. Due to the fact that small shifting of resonatorcharacteristics results in large difference in the set of lasingwavelenghts, an optical laser having a wide tunability, is obtained.Said laser is therefore denoted widely wavelength tunable integratedsemiconductor laser.

According to a preferred embodiment of the present invention, the activeradiation-generating section is connected at one side to a plurality ofgrated sections, but said gratings are not included in said activesection. Moreover the gratings are selected such that substantialinteraction between the spectra of said gratings can be used becausethis is the working principle used for improving the selectivity.Therefore at least one of said resonant maxima of each of two saidsections are overlapping with each other.

In an embodiment of the invention, only one of said resonator maximapoints is overlapping. It can then be said that said active sectioncreates a radiation or a lightbeam by emission and the device emits anemitted laser beam with the wavelength of said emitted lightbeam beingin correspondence with said overlapping maxima of said transmissionfilters or reflectors. The combination of said plurality of sections,having a combinated reflection action with a single reflectionwavelength, and said (optical) amplification action of said activesection causes lasing at said single reflection wavelength, defined bysaid overlapping resonator maxima points.

In an embodiment of the invention, at least one of said plurality ofsections is inactive. This means that such inactive section is notcreating a lightbeam by emission.

In an embodiment of the invention, at least one of said plurality ofsections is active. This means that such active section also creates alightbeam by emission.

In an embodiment of the invention, at least one of said waveguidesystems has a periodically broken short-period structure including shortperiod stripped regions alternating with non-stripped regions. Suchwaveguide systems are also denoted as distributed, hence said laser isdenoted a semiconductor laser with distributed reflection ortransmission sections. In one aspect of the invention, two suchwaveguides can be found on the same side of the active region.

In an embodiment of the invention, at least one of said waveguidesystems has a diffractive grating having a plurality of repeating unitregions each having a constant length, thus forming a modulation period,and at least one parameter that determines the optical reflectivity ortransmission of said diffractive grating varying depending on itsposition in each of said repeating unit regions along a direction ofoptical transmission in said laser, said diffractive grating extendingby at least two modulation periods.

In an embodiment of the invention, at least one of said waveguidesystems is a ring resonator.

In an embodiment of the invention, the laser further comprises aplurality of power splitters, being exploited for optically connectingpart of said plurality of sections and connecting part of said pluralityof sections with said active section.

In an embodiment of the invention, said laser is a serial concatenationof said active section and a plurality of said sections.

In an embodiment of the invention, said laser is a connection of saidactive section to one single port side of a power splitter and aparallel connection of a plurality of sections to the other multi portside of said power splitter.

In an embodiment of the invention, said laser comprises phase sections,being exploiting for adjusting the round trip cavity phase and thus alasing mode wavelength of the laser.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal cross-section of an SG-DBR laser. As opposed toa conventional DBR laser, the reflectors are formed by two periodicallymodulated (sampled) gratings with different sampling periods.

FIG. 2 is a schematic diagram of a GCSR laser structure.

FIG. 3 is a top view (schematic) of the proposed “Y-SSG” laser.

FIG. 4 is a schematic view of a ring resonator (S)SG laser.

FIG. 5 is a transmission characteristic of a ring resonator.

FIG. 6 is a principle scheme of the laser according to one aspect theinvention, comprising an active element and a plurality of sections,being either transmission filters or reflectors.

FIG. 7 is a principle scheme of prior-art lasers, comprising an activeelement and reflectors, said active element being bounded by saidreflectors.

FIG. 8 is a principle scheme of the second embodiment of the invention,comprising an active element and a plurality of sections, beingparallel.

FIG. 9 is a principle scheme of the fourth embodiment of the invention,comprising an active element and a serial concatenation of transmissionfilters ended by a reflector.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

In the further description, several embodiments of the invention arepresented. It must be clear that the scope of the invention is describedby the claims.

The present invention discloses alternative laser structures, which havepotentially the same tuning performance as (S)SG-DBR and GCSR lasers andwhich the output power does not pass through a long passive region. Insaid devices, there are passive or inactive regions on one side of theactive region only.

A principle scheme of prior-art conventional Distributed Bragg Reflector(DBR) semiconductor lasers is shown in FIG. 7. A schematic descriptionof such a laser is shown in FIG. 1, showing a front reflector 500, arear reflector 510, an AR (anti-reflective coating) 520, an activesection 530 and a phase section 540. Tuning of said laser is limited bythe fact that the relative tuning range 360, as shown in FIG. 7, isrestricted to the relative change in the refractive index of the tuningregion. This means that the tuning range, under normal operatingconditions, cannot exceed 10 nm. This is substantially less than thepotential bandwidth, restricted by the width of the gain curve 350,which is about 100 nm. Such conventional DBR lasers can functionally becharacterized as comprising a first part being a two-sided.activesection 300, for creating a light beam 310 by spontaneous emission overa bandwidth around some center frequency 370, as observed in said laserscharacteristic 340. Said first part also guides said light beam. Suchconventional DBR lasers further have two reflectors 330, 320. Saidreflectors are bounding said two-sided active section 300, thus one ateach side.

A classical laser structure comprises (i) a two-sided activesection/region, creating a light beam by spontaneous emission over abandwidth around some center frequency and guiding said light beam, saidactive section performing optical amplification actions and (ii) two(inactive or passive) sections/regions, acting as reflectors. Saidactive section is bounded by said two reflectors.

Besides (inactive or passive) reflecting sections/regions, also sectionswith a transmission characteristic exist.

The invention in one embodiment can be characterized as comprising atwo-sided active section/region (just as a classical laser) and aplurality of sections/regions. Said plurality of sections/regionsdefines a network of sections/regions. Said network of sections/regionsis connected to one side of the active region. Said network comprises atleast two resonator regions/sections. Said resonator regions/section canbe either reflectors or regions/sections with a transmissioncharacteristic. The principle scheme of the laser according to theinvention, comprising an active element 30 and a plurality of sections50, 90, 100, 110, being resonators, thus being either transmissionfilters or reflectors, is shown in FIG. 6.

The invention in one embodiment can be characterized as a devicecomprising (i) a substrate made of a semiconducting material, (ii) atwo-sided active section on said substrate, said active sectiongenerating radiation by spontaneous emission over a bandwidth aroundsome center frequency and guiding said radiation, said active sectionhaving amplification actions, and (iii) a plurality of sections on saidsubstrate, all said sections being connected to one side of said activesection, at least two of said sections including a waveguide system,defining either a transmission filter or a reflector. The device canfurther comprise a plurality of power splitters, being exploited forinterconnecting part of said inactive sections and connecting part ofsaid sections with said active section. Said sections can be eitheractive or inactive.

When said device creates a light beam, said connection of said networkwith said active section is an optical connection. Said connections ofsaid sections are then also optical. Said device can then be denoted asa tunable integrated/semiconductor optical laser. Said amplification isthen denoted to be an optical amplification. Said active section thenalso guides said light beam.

In one embodiment of the invention, particular reflectors and sectionswith transmission characteristics are exploited. Said reflectors andsections with transmission characteristics are commonly denoted asresonators. Said reflection and transmission sections are functionallycharacterized as having a reflection or transmission characteristic witha plurality of reflection or transmission peaks, commonly denoted asresonant peaks. Said reflection or transmission characteristic hasspaced reflection or transmission maxima points providing a maximumreflection or transmission of an associated wavelength. The resonatorcharacteristic thus has a plurality of spectral response peaks,preferably narrow spectral response peaks. Said resonator characteristiccan be either regular, meaning that its resonant frequencies are allspaced apart by a same value, being periodic, or irregular, meaning thatthere is no fixed spacing between its resonant frequencies. Irregularitycan be a random pattern of resonant frequencies or some structuredpattern.

Such a characteristic can be obtained via sampled gratings, whichexhibit a comb-shaped reflection or transmission spectrum or via theso-called supergratings. Said gratings or supergratings can also becharacterized as distributed reflectors or transmission sections.

Sampled gratings can be described as structures in a waveguide systemhaving a periodically broken short-period structure including shortperiod stripped regions alternating with non-stripped regions. Thesupergratings can be described as structures in a waveguide systemhaving a diffractive grating having a plurality of repeating unitregions each having a constant length, thus forming a modulation period,and at least one parameter that determines the optical reflectivity ortransmission of said diffractive grating varying depending on itsposition in each of said repeating unit regions along a direction ofoptical transmission in said laser, said diffractive grating extendingby at least two modulation periods.

In one aspect of the invention, alternative sections with transmissioncharacteristics based on ring resonators are exploited. Such a ringresonator has a comb-shaped transmission characteristic (FIG. 5). Theoperation of a ring resonator is similar to that of a classicFabry-Perot resonator, which can easily be understood if one thinks ofthe cross-coupling of the two couplers in the ring as being thetransmission mirrors in the FP-resonator.

In the prior art, constructive interference of the periodiccharacteristics of reflectors, located at different sides of the activesection, with different periodicity is exploited to obtain a widetunability. As the construction of said reflectors leads to longinactive sections, this results in lasing output power losses.

In one aspect of the invention, constructive interference of theperiodic characteristics of sections, being either reflective ortransmissive, and with different periodicity and located at the sameside of the active section is exploited. This approach results in widetunability of said laser. Even when said reflectors are by constructionlong inactive sections, this is not harmful for the lasing output power,as these are only located at a single side of the active section. Theinvention results in a low loss window at any time.

Thus the invention can be further characterized by stating that thespacing of at least two transmission or reflective maxima points of therespective (inactive or even active) sections being essentially notequal or different, and at least one said transmission or reflectivemaxima of each of said (inactive or even active) sections being incorrespondence with a wavelength of said created lightbeam. Thiscorrespondence is obtained by having at least one of said resonant peaksof at least two sections being overlapping or coinciding.

The combination of said plurality of sections can be considered as acombined reflector, having a combination reflection action. Said opticalamplification action of said active section and said combinationreflection action of said combined reflector are causing lasing at atleast one of the reflection wavelengths of said combined reflector.

FIG. 6 shows an example of a possible configuration, although theinvention is not limited hereto. The active section 30 is optically atone side connected with a plurality of sections 120, said plurality ofsections comprising a transmission filter 50 and three reflectors 90,100, 110. The lasing light 10 leaves the laser at the other side of theactive section 30. Numeral 20 indicates the amplification action withinsaid active section 30. Said connections 40, 60, 70, 80 indicate theoptical interconnectivity and should not be considered as physicalconnections.

The transmission filters exploited in the invention have a transmissioncharacteristic 150 with a plurality of transmission peaks.Alternatively, it can be said that said transmission filters have spaced(spacing 130) transmission maxima points providing a maximumtransmission of an associated wavelength. The reflectors exploited inthe invention have a reflection characteristic 160 with a plurality ofreflection peaks. Alternatively it can be said that said reflectors havespaced (spacing period 140) reflective maxima points providing a maximumreflection of an associated wavelength. The spacing 130, 140 of saidtransmission or reflective maxima points of at least two of saidsections are selected to be different. Said laser is therefore denotedas an integrated semiconductor laser with different reflection ortransmission sections. The transmission and reflection characteristic ofsaid transmission filters and reflectors are positioned such that onlyone of said transmission or reflective maxima of each of said sectionsis overlapping, meaning having a transmission or reflective maxima forthe same frequency. Due to the different spacings of said transmissionor reflective maxima points, a small shift of one of said transmissionor reflective maxima points results in an optical laser having a widetunability. Said laser is therefore denoted a widely wavelength tunableintegrated semiconductor laser. Said shift can be due to currentinjections in said transmission or reflective sections. It can be saidthat said laser comprises means for injecting current into part of saidplurality of sections, resulting in said transmission or reflectioncharacteristic being shifted in wavelength.

The configuration of FIG. 6 can, for instance, be achieved by using apower splitter 200 for connecting section 50 with sections 90, 100, 110.Said power splitter has a single port side (side connected to 50) and amulti port side (side connected to 90, 100, 110, with three ports.

A schematic view of a prior art GCSR laser structure is shown in FIG. 2,showing an active section 600, a coupler section 610, a phase section620, and a reflector 630. FIG. 2 further shows the cross sections ofsaid sections. Said coupler section 610 and said reflector section 630have an essentially different resonance characteristic. Said couplersection 610 does not have spaced resonant maxima points.

In a first embodiment, a Y-structure as in FIG. 3 is proposed with tworeflectors 710, 720, placed on the same side of the active region 700.These two reflectors use sampled or super structure gratings to providereflection combs with different periods; their design is the same as for(S)SG-DBR lasers. The power splitter 730 is used to split/combine thelight leaving/entering the active region 700. Two phase control sections740, 750 are shown in FIG. 3. One provides the correct phase relationbetween the signals reflected by the two reflectors, whereas the secondprovides control of the overall phase of the combined reflected signal.According to an alternative embodiment, a separate phase control sectioncould be placed in each of the arms of the Y. The branches of theY-structure could be all-active, thus avoiding a (technologically morecomplex) transition from an active waveguide to a passive one. However,this has the disadvantage that the device will be more difficult tocontrol, because power and wavelength control will be mixed. Therefore,passive waveguides for both branches of the Y-structure are preferred.

In a second embodiment, a second structure, which in a top view lookslike in FIG. 8, is proposed. In this structure, a plurality ofreflectors 440 (including reflectors 410, 420, 430) are placed on thesame side of the active region 400. At least two of said reflectors usesampled or super structure gratings to provide reflections combs. Atleast two of said reflectors have reflection combs with differentperiods. The design of said reflectors is the same as for (S)SG-DBRlasers. The power splitter 450 is used to split and combine the lightleaving/entering the active region 400. Phase control sections can beintroduced. In one configuration one phase control section is placedbetween the active region and the reflectors and in all reflectors,except one, also a phase control section is provided. In anotherconfiguration only phase control sections are provided in saidreflectors.

In a third embodiment, a third structure, as shown in FIG. 4, isproposed. The reflector consists of an active section 830, a ringresonator 800, which has a comb-shaped transmission characteristic (oneexample is shown in FIG. 5) and a (S)SG-reflector 810 with, in oneembodiment, anti-reflective coating 820. The operation of a ringresonator is similar to that of a classic Fabry-Perot resonator, whichcan easily be understood if one thinks of the cross-coupling of the twocouplers in the ring as being the transmission mirrors in theFP-resonator. The tuning is again based on the “Vernier”-principle, asin conventional (S)SG-DBR lasers: the ring resonator and the(S)SG-reflector are designed to have slightly different peak-spacing intheir transmission and reflection characteristics respectively, andlasing occurs at or near the wavelength where two peaks overlap eachother. A phase section, used to align a longitudinal cavity mode withthe two aligned peaks, could also be included in this structure. Itcould be placed either between the active section and the ringresonator, or between the ring and the (S)SG-reflector.

In a fourth embodiment, a fourth structure, as shown in FIG. 9, isproposed. This structure comprises an active region 900 bounded at oneside by a serial concatenation of sections, being a plurality oftransmission filters 940, including filters 910, 920, said concatenationends with a reflector 930. At least two of said sections, the sectionsbeing either transmission filter or reflector, have a comb-shapedtransmission characteristic. At least two of said sections, the sectionsbeing either transmission filter or reflector, are designed to have aslightly different peak-spacing in their characteristic. A phase sectioncan be placed between any of said sections and between said activesection and said sections. Said transmission filters can be either(S)SG-transmission filters or a ring-resonator. Said reflector is a(S)SG-reflector.

A main feature of the proposed Y and ring-structures and any structureexploiting a combination of the principles on which the proposed Y andring structures are based is that light is emitted directly from theactive region without going through a passive region. This is expectedto result in a higher efficiency and a lower degree of power variationduring tuning. Typical reflectors can be designed to have a uniformenvelope of the reflection peaks. These designs require long passivewaveguides and the reflectivity at the reflection peaks is quite high.Both of these factors lead to a reduced efficiency in a two-sidedreflector structure. In all proposed structures, the output power is notpassing through a reflector, hence a high efficiency can be expected,and the high reflectivity is now an advantage. Therefore, in one aspectthe invention, all of said sections are optically connected to one sideof said active region.

Since the reflectors are preferably passive, the Y structure (of thefirst embodiment) and the structure of the second embodiment, do notsuffer from the control problems present in the Y-laser known fromprior-art.

The performance with respect to tuning and output power is expected tobe similar to that of a GCSR laser, but the fabrication is easier. Thenumber of process steps will be the same as for an SG/SSG, but lowerthan for a GCSR laser. Except for mask design, the fabrication is nearlyidentical to that for SG/SSG DBR lasers, but AR coating of the facetsmay not be necessary when the grating is designed for high peakreflectivity. As an alternative, one could use an absorbing region tokill off any unwanted reflection from the facet.

There may be some radiation losses associated with the power splitter inthe Y-laser, but these are unlikely to be higher than the radiationlosses in the tunable coupler section of the GCSR.

It is intended that the foregoing detailed description be regarded asillustrative rather than limiting and that it is understood that thefollowing claims, including all equivalents, are intended to define thescope of the invention.

1. A widely tunable laser apparatus comprising: a substrate comprising asemiconductor material; at least two resonator sections formed on thesubstrate, wherein each of the at least two resonator sections compriseone of a transmission filter and a reflector; and a two-sided activeradiation-generating section formed on the substrate, the at least tworesonator sections being coupled with a single side of the two-sidedactive section, wherein each of the at least two resonator sectionscomprises a waveguide system, each waveguide system operatively havingspaced resonant maxima points, so as to provide one of a maximumtransmittance and a maximum reflectance when subjected to energy of afrequency corresponding with one of the resonant maxima points, andwherein at least two spacings of the plurality of resonant maxima pointsare differently spaced in the frequency domain for at least two of theresonant sections.
 2. The apparatus of claim 1, wherein the spacing ofthe plurality of resonant maxima points in the frequency domain isdifferent for at least two of the resonator sections.
 3. The apparatusof claim 1, wherein, at least one of the resonant maxima points of eachof the at least two resonator sections are adjustably overlapping. 4.The apparatus of claim 1, wherein the active section creates a lightbeam as a result of spontaneous emission over a bandwidth around acenter frequency and guides the light beam.
 5. The apparatus of claim 1,wherein the active section creates a light beam as a result ofspontaneous emission over a bandwidth around a center frequency andoptically amplifies the light beam.
 6. The apparatus of claim 5, whereinthe apparatus produces a combined reflective action and the opticalamplification causes lasing at at least one of the wavelengthsassociated with one of the reflective maxima points.
 7. The apparatus ofclaim 1, further comprising a power splitter for coupling one or more ofthe at least two resonator sections with the active section.
 8. Theapparatus of claim 7, wherein the power splitter is coupled with theactive section via a first side of the power splitter having a singleport and coupled with the at least two resonator sections via a secondside of the power splitter having a plurality of parallel connections.9. The apparatus of claim 1, wherein only a single resonant maxima pointof each of the at least two resonator sections overlap.
 10. Theapparatus of claim 1, further comprising one or more phase controlsections coupled with at least the active section for adjusting a roundtrip cavity phase of the apparatus.
 11. The apparatus of claim 1,further comprising one or more phase control sections coupled with atleast one of said two resonator sections for adjusting a round tripcavity phase of the apparatus.
 12. The apparatus as recited in claim 1,further comprising a current source coupled with the at least tworesonator sections for injecting current into one or more of the atleast two resonator sections, so as to cause one of a transmissioncharacteristic and a reflection characteristic to be shifted inwavelength.
 13. The apparatus of claim 1, further comprising one or morephase control sections coupled with one of the active section and the atleast two resonator sections, wherein the phase control sections areemployed to adjust a round trip cavity phase of the apparatus.
 14. Theapparatus as recited in claim 13, further comprising a current sourcecoupled with the one or more phase sections for injecting current intoone or more of the phase sections, so as to cause the roundtrip cavityphase of the apparatus to be adjusted.